Ideal HOW Acoustics

How variable acoustics can create a continuously adaptive sound environment for contemporary houses of worship.

A section view showing locations of speakers and microphones for a reverberation enhancement system. The overhead speakers are ceiling-mounted and the mics are hung down from the ceiling. The lateral speakers are wall-mounted.

One of the most famous lines in American film is found in the movie The Graduate, when young Dustin Hoffman is told, "I've got just one word to say to you about the future: Plastics." Let's move to the present and let's limit our focus to sound in multipurpose spaces such as theaters and houses of worship. I've got just two words (and a hyphen) to say to you about the future: Electro-acoustic architecture.

What would be the ideal room acoustics in a contemporary house of worship? Of course the normal answer would be, "It depends." Now we have a better option. The best answer is acoustics that don't depend. The best answer is acoustics that can adapt in realtime to the current needs. If the choir is singing, give us a cathedral. If the band is rocking, let's keep it tight in the low-end, and if there is a speech in progress, let's give the voice just enough support to be easily heard without excessive reverberation. Is there any kind of hall that has these contradictory acoustical properties? Yes—a hall with variable acoustics. Halls with variable acoustics are a tiny minority at the present time. Houses of worship are a small minority of that minority. But in the not-too-distant future, variable acoustics will be commonplace for houses of worship.

Variable acoustics come in two forms: mechanical and as electro-acoustic enhancement systems. The mechanical version has been used effectively in the construction of many of modern era symphony halls. These halls look like a typical symphony hall on the inside, but they contain more hidden doors, chambers, curtains, and secret passages than Hogwarts School of Witchcraft and Wizardry. Such a hall can have its reverberation time extended by opening chambers or conversely have it reduced by dropping in curtains or reversing doors from a reflective to an absorbent side. A modern symphony hall can be optimized for classical one night (1.8 seconds of reverberation time), for pipe organ the next (5+ seconds), and then bring in a lecture presentation (1.0 seconds).

Adding mechanical variable acoustics is a huge up-front cost to a modern symphony hall, but it is orders of magnitude cheaper than building three halls. The operational and maintenance costs are substantial as well, but this is more than made up for by the fact that the variable hall can be booked with a much wider variety of acoustically successful events than any hall with static acoustics. This means more bookings and fewer dark days. The variables at play here are volume and the amount of absorption.

Increased volume (for a given amount of absorption) will increase the reverberation. Increased absorption (for a given volume) will decrease the reverberation. The variable acoustic hall in its "standard" configuration for symphonic music has a large proportion of exposed reflective surfaces to create an even decay from all directions. These surfaces cannot be made more reflective, so in order to increase the reverberation, we must increase the volume. Doors are opened to allow the sound into highly reflective coupled chambers that increase the volume of air in the space, thereby extending the decay time.

Conversely, if we want to reduce the reverberation, it is much more practical to increase the absorption. This is done by dropping in curtains or turning panels over to their absorptive side or other mechanical options. The contemporary house of worship does speech, organ, rock, choir, rap, and more. Therefore variable acoustics is a natural fit. But, unlike the concert hall, the HOW does not do speech one night and choir the next. The entire range of musical genres and speech may all occur in an hour or less. How many volunteers will it take to operate the reversible doors, open and close chambers, and drop down curtains for each different part of the service? Do you think this might be just a bit distracting to one's spiritual contemplation?

Obviously this is not practical, which brings us to the second option in the world of variable acoustics: electro-acoustic architecture (also known as acoustic enhancement systems, artificial reverberation, reverberation enhancement, and other names). With electro-acoustic architecture, you can change the room's response from voice-optimized to organ-ready in a single second, without any visual clue and without a labor call.

Whereas the mechanical variable acoustics began in the middle acoustical ground and added volume or absorption to go one way or the other, the electro-acoustic technology can only add reverberation, not reduce it. Therefore the electro-acoustic versions want to start from a relatively dry room and then add from there. A well-designed system can at least double the reverberation time of the physical room and possibly extend it further from there. This is achieved by electronically simulating the effects of adding volume and decreasing absorption.

How electro-acoustic architecture works (simplified)

As the name implies, the acoustic response is going to be enhanced electronically. There are three major ingredients: microphones to simulate the sound going to the walls, speakers to simulate the reflections coming off the walls, and digital signal processing to manage the character of the reverberation created and possibly to create additional electronic reverberation. All of this is added on top of the original acoustical properties of the space. Reduced to its simplest form, we have the following signal path: (A) acoustic source to (B) mic to (C) processing to (D) speaker, which then returns to the mic and around we go again and again. This is similar to the natural acoustic path: (A) acoustic source to (B) wall, (C) wall, (D) wall, and on and on. This contrasts to the reverberation device you might have out at front of house, which creates all its reverberation inside the processor and then sends it out to the speaker system.

Now let's dig deeper and get some more detail about what it is going to take to create a plausible multidirectional reverberation tail in a dry room. If we just add reverb into the speaker system, we have reverberant speakers in a dry room. The more reverb we add the more out of scale the two worlds become. I like to put it to the "singing in the shower" test. Electronic reverb puts the singer in the shower, but the audience is still dry. This could make folks feel a bit uncomfortable.

If the reverberation is created in the room by either physical acoustics or electro-acoustic architecture, then the audience and the performer are all in the shower together. Much better, eh?

The key ingredients that separate reverberation created in the room from your front-of-house electronic reverberation are that the in-room reverberation is initiated by any sound source, anywhere in the room, and that the reverberation surrounds you. Your FOH reverb affects only the performer(s) and only arrives from the direction of the speakers, which are typically in front of you. How about if we add some reverb to the surround speakers? Nice try, but surround speakers are too few and far between to fool anybody into believing that they are in a reverberant space rather than hearing a few isolated effects sources.

Reverberation enhancement systems use multiple mics placed around the stage and audience areas as well as large number of speakers along the walls and ceiling. The density of the speaker placement has to be high enough to make the listeners unable to localize individual elements, thereby creating the unified reflective character of a wall. The quantity of mics has to be high enough to give us lots of return portals into the speakers so that we can have enough recirculated energy to extend the reverberant energy without causing instability and feedback. The quality of all components must be high enough so that no clues are given to listeners that the sound is coming from speakers or mics.

Ideal HOW Acoustics

How variable acoustics can create a continuously adaptive sound environment for contemporary houses of worship.

A house of worship using electro-acoustic architecture. The mics are suspended from the ceiling and the bridge above the stage. Overhead speakers are located on the ceiling and the bridge. Lateral speakers are found on the side and rear walls. The sound operators modify the reverberation settings during the course of the service.

Simulating the acoustic space (simplified)

An acoustic space creates a very complex layering of reflections. Every location has a unique series of individual arrivals, yet taken as a whole the number of reflections and decay time are largely uniform over the space. A well-behaved decay structure will have sufficient density and variability so that individual reflections cannot be localized. The level of the decay tail should fall steadily over time. One of the critical failures in physical acoustics is when reflections are heard with evenly spaced intervals, known as flutter echoes, which are easily detected by our ears. Another failure is when a single reflection stands out above the crowd, a specular reflection, which draws the listener's attention to a single spot in the room.

The job of the reverberation enhancement system is also to increase the density at all locations evenly, just as would occur with reduced absorption and added volume without allowing for evenly spaced reflection intervals or having any single speaker stand out on its own. We have the additional constraint of having to ensure that our system does not do any of the other things that would blow our cover such as feedback, hum, noise, rattles, distortion, or impossible events such as a reflection arriving ahead of the direct sound. A key factor that makes it possible and practical to simulate a reflective wall is the use of large numbers of separate signal channels.

A side view of the stage. Look carefully and you can see the miniature mics just below the “M” marking.

Whereas your usual surround speakers might all run on a single feed from the console, the lateral speakers that create our side wall "reflections" will each be driven with unique channels of processing. The input into each speaker comes from the mics, but each speaker gets a different mix of the multiple mics. This creates a unique semi-correlated signal for each speaker feeding back into the space. It might not be intuitive that this simulates a physical wall, but consider this example: When there are 20 violins on stage, the signals all originate from unique locations. You will hear each of their reflections arriving from different locations off the walls and ceilings. No two sets of reflections follow the same paths, although neighboring instruments will track closely. To our ears, the violin reflections are spread along the wall rather than concentrated to a single spot. Our signal processing will create the same effect by uniquely blending the arrivals of the different mics with randomized timing and level differences. So now we get to the heart of the matter.

How does this really work? The first thing to understand is that the input signal is acoustic. It begins with sound arriving at our mics in the room (not close mics on stage). Maybe the sound came direct from a performer, from the PA speakers, or even from the audience. The electro-acoustic architecture re-circulates every sound source just as a wall would. So let's take a very basic system with 16 mics, 24 speakers along the walls (laterals), and eight overhead speakers. The original sound goes into the room and arrives at all 16 mics but each at slightly different times (due to path length) and with varying frequency responses (due to directionality of the source and mics).

The 16 signals are then routed into the room acoustic signal processor(s) and mixed together in a complex matrix which then is sent to the speaker outputs. Each output then has a unique blend of the mics and this can be sent to the speakers. Since the sound leaving each speaker is a mix of multiple timings, the source appears much more diffuse (and therefore like a wall) than a speaker reproducing the sound from a single mic. Because the neighboring speakers are reproducing non-identical signals, the blend between the adjacent speakers is much more gradual than would be the case with correlated signals (like your traditional surrounds). Round one is complete. The next thing that occurs is that the sound from the speakers hits the mics. Now each mic hears every single speaker, each of which are sending a diffuse mix of the 16 mics. Then back to the variable room processor we go, this time with signals more dense than last time, and the cycle continues. In some systems, the processor can optionally add internally generated reverberation to the matrix mix. In some systems the outputs have varying transit time through the device to help suppress feedback.

In practice, I have seen these systems put to use in houses of worship on a cue-by-cue basis through the course of the service. The changes are as simple as clicking the mouse to select one of the settings from the library of configurations. These are operated by mix engineers, not rocket scientists or acoustic consultants. When done well, the audience simply accepts that the sonic experience seems appropriately adapted to the music or speech and becomes very involved in the service. And if you want the congregation to sing along, for God's sake surround them with a lot of reverb.

The systems

This is just the tip of the iceberg in terms of the complexity and capability of these rapidly evolving systems. The capabilities described here are not necessarily universal to all of the systems available on the market. A feature by feature comparison of the different systems is not possible without getting much deeper into the math, and therefore will be saved for another article (and another author).

These are the main systems out there that I know of and you can get lots of information about them online: Meyer Sound's Constellation, The System for Improved Acoustic Performance (SIAP), Lexicon Acoustic Reinforcement and Enhancement System (LARES), and Yamaha's Active Field Control (AFC).

Bob McCarthy is president of Alignment and Design. McCarthy specializes in the design and tuning of sound reinforcement systems and conducts trainings around the world. His book, Sound Systems: Design and Optimization, was named "2007 Sound Product of the Year" by Live Design. Visit his blog at bobmccarthy.wordpress.com.